A BIN price is a price at which a seller is happy to sell the property on the spot. If a buyer is willing to pay this amount, the seller is willing to end the auction immediately and declare the buyer the winner.
The BIN price must be at least as high as the reserve price, but it can be higher.
It should reflect the maximum price the seller believes they can get for the property, because if a BIN price is set on a property, the property won't sell for more than that price.
A seller may set a BIN at any time during a listing.
See How do Buy It Now (BIN) bids work? for more information on BIN bidding.